The invention relates to a method for generating radiation with stabilized frequency, in particular a method for generating stabilized frequencies in the range from radio frequencies up to optical frequencies, and to a device for generating radiation with stabilized frequency, e.g. a stabilized laser generator.
The generation of short light pulses (typical pulse widths in the range down to ps or fs) with laser sources is generally known for many years. As an example, light pulses are generated by locking of oscillation modes of the light field in the laser resonator (mode locking). The light pulses are emitted as a periodic pulse train with a pulse-pulse-distance or repetition time T=fRxe2x88x921 (see FIG. 1). The repetition time is determined by the length L of the laser cavity and the mean group velocity of the pulses Vgr according to T=2L/Vgr. The consideration of the pulses in the frequency domain yields a frequency comb with a mode structure illustrated in the lower part of FIG. 1. The frequency comb comprises a series of frequency components according to the number of oscillation modes contributing to the pulse formation. The distance of the frequency components equals the repetition frequency fR.
Due to the difference between the phase velocity of the single frequency components and the group velocity associated with the pulses, a phase shift xcex94xcfx86 appears causing a shift of the absolute frequencies fN of the components according to fN=n fR+f0 with f0=xcex94xcfx86/T2xcfx80. The repetition frequency fR and the offset or slip frequency f0 represent two degrees of freedom of the frequency comb.
For current applications of laser light, in particular ultra-short light pulses, e.g. in the fields of spectroscopy, telecommunication and time measurement, highly stabilized optical frequencies and pulse parameters are required. While the repetition frequency fR can be measured and controlled by adjusting the cavity length of the laser resonator, offset frequency f0 requires particular control techniques.
T. Udem et al. (xe2x80x9cPhys. Rev. Lett.xe2x80x9d, vol. 82, 1999, p. 3568) disclose the stabilization of frequency combs by introducing a linear dispersion into the resonator. This technique allowing a control and stabilization of the offset frequency f0, suffers from the following disadvantage. The control of the first order dispersion in the cavity requires a sensitive measurement and adjustment system which is difficult to be handled under practical conditions, e.g. in a routine spectroscopic measurement arrangement or in a pulse source for telecommunication applications.
The object of the invention is to provide an improved method for generating radiation with stabilized frequency.
It is another object of the invention to provide an improved radiation source for generating radiation with stabilized frequency, e.g. a stabilized laser light or microwave generator.
The method for generating radiation with stabilized frequency comprises the steps of providing laser light pulses with a repetition frequency fR, the pulses comprising a plurality of N frequency components fn with fn=nxc2x7fn+f0, wherein f0 represents an offset frequency with n=1, . . . , N, the frequency components forming a comb with first and second different frequency portions, and generating a primary light output with at least one output frequency component corresponding to the difference of frequencies of the first and second frequency portions
According to one embodiment of the invention, the first and second different frequency portions are delivered directly to a difference frequency generator. According to another embodiment, a method for generating radiation with stabilized frequency is provided, wherein the laser light pulses with a repetition frequency fR are subjected to a frequency filter device transmitting radiation comprising first and second filtered portions corresponding to different first and second frequency intervals xcex4f1, xcex4f2, each of the filtered portions comprising at least one frequency component fN,1, fn,2, and the primary light output with at least one output frequency component is generated corresponding to the difference of frequencies of the first and second filtered portions.
The laser light pulses comprise a plurality of N frequency components fn with fn=n fR+f0, wherein f0 represents an offset frequency with n=1, . . . , N. The laser pulses can be produced by any available pulse laser. Preferably, the pulses have a mean pulse width in the range of 10 ps to 10 fs or lower and a repetition frequency fR in the range of 20 MHz to 3 GHz. The primary light output is generated using a non-linear difference frequency generation process being known as such. For obtaining a primary light output with sufficient output power, each of the first and second filtered portions comprise a plurality of frequency components with N=1000 or higher, However, the invention can be implemented with smaller N-values, even with N=1, if cw lasers are phase locked to single frequency modes of the laser pulses (see below) and the frequency difference of the cw laser outputs is provided as the primary light output and/or if the Signal/Noise-Ratio is sufficient high.
An essential advantage of the invention is presented by the fact that the primary light output comprises at least one frequency component or up to M frequency components fm with fm=m fR with m=1, . . . , M. The frequency component(s) (or a mean frequency) are independent of the offset frequency. The non-linear frequency difference process eliminated the offset frequency which is contained in both first and second filtered portions of the laser light pulses. Furthermore, the frequency components fm can be completely adjusted by controlling the repetition frequency of the pulse laser only.
According to a preferred embodiment of the invention, the laser pulses are subjected to non-linear self phase modulation before the filtering for broadening the frequency comb corresponding to the pulses. The broadening preferably is obtained by transmitting said laser light pulses through a non-linear optical element producing further frequency modes, as e.g. an optical fiber. Optical fibers with a strong comb broadening, as photonic crystal fibers (see D. Mogilevtsev et al. in xe2x80x9cOptics Lettersxe2x80x9d, vol. 23, 1998, p. 1662, T. A. Birks in xe2x80x9cOptics Lettersxe2x80x9d, vol. 22, 1997, p. 961, or T. A. Birks in xe2x80x9cIEEE Photonics Lettersxe2x80x9d, vol. 11, 1999, p. 674), are preferably used.
Another subject of the invention is the provision of a radiation source device for generating radiation with stabilized frequency, the radiation source device comprising in particular a laser pulse generator for generating the laser light pulses with a repetition frequency fR, optionally a frequency filtering device for providing the first and second filtered portions, and a non-linear difference frequency generator being adapted to generate a primary light output with at least one output frequency component corresponding to the difference of frequencies of the first and second frequency portions or filtered portions.
According to a preferred embodiment of the radiation source device, a pulse broadening device (e.g. an optical fiber) is arranged between the laser pulse generator and the frequency filtering device.
According to a further preferred embodiment of the invention, a laser or a microwave generator is phase-locked to the primary light output for generating a secondary light output with a stabilized optical or rf frequency, respectively.
A particularly important advantage of the invention results from the broad application range of the generated primary light output. Depending on the laser light pulse width, the first and second filtered portions may have a high frequency distance ranging up to optical frequencies, or a small frequency distance corresponding to radio frequencies, The radiation source device provides an universal frequency generator for a series of applications in time/frequency measurements (e.g. spectroscopy) and in telecommunications.
The invention has the following further advantages. The radiation source device according to the invention has a compact and robust design allowing an easy adjustment and applications under routine conditions. The radiation source device is compatible with conventional laser optical beam forming and measurement techniques.